Lymphocytes from approximately 50% of individuals infected with HTLV-II proliferate spontaneously in vitro in the absence of any added stimulus, observations drawn upon studies of HIV-seronegative injection drug users (IDU). This project is examining factors that contribute to this phenomenon and its relevance to disease progression. In the present study we have examined spontaneous lymphocyte proliferation (SLP) in IDU who are infected with HTLV-II alone or are co-infected with HIV, and IDU controls. Ten individuals were examined, of whom 2 were seropositive for both HTLV-II and HIV, 2 were seropositive for HTLV-II alone, 2 were seropositive for HIV alone, and 4 were seronegative for both HTLV-II and HIV. Gradient purified peripheral blood lymphocytes (PBL) were cultured for 7 days and analyzed for SLP by pulsing with 3H-thymidine and for expression of viral RNA by RT-PCR. Viral expression was not detectable in any of the samples prior to the culture, nor in post culture cells derived from uninfected patients or individuals infected with HIV alone. Following the 7 day culture period, however, viral transcripts could be detected in all 4 of the HTLV-II positive samples. In contrast, SLP was detected only in those samples that were derived from individuals infected with HTLV-II alone. These results demonstrate that expression of HTLV-II in cultured PBL from infected individuals is not sufficient to result in SLP. Furthermore, the results suggest a negative correlation between co-infection with HIV and the ability of PBL from HTLV-II infected individuals to exhibit SLP. This, in turn, indicates that HIV itself, or cells that it influences, may actively interfere with the SLP phenomenon.